In order to unlock your classes advanced job you must first reach level 30 in your main class and level 15 in the sub class. Once these requirements are met you must complete the quest given by your the guild for your main class.

Job	How to unlock
Bard	Archer 30 / Pugilist 15
Black Mage	Thaumaturge 30 / Archer 15
Dragoon	Lancer 30 / Marauder 15
Monk	Pugilist 30 / Lancer 15
Paladin	Gladiator 30 / Conjurer 15
Scholar	Arcanist 30 / Conjurer 15
Summoner	Arcanist 30 / Thaumaturge 15
Warrior	Marauder 30 / Gladiator 15
White Mage	Conjurer 30 / Arcanist 15